For many medical purposes, it is important to place devices, including soft, yielding devices, through the skin and underlying tissue layers and into blood vessels or other compartments or locations inside the body of a medical patient. These include but are not limited to catheters, pacemaker leads and hydrocephalic shunts. Such devices often have one end in the desired location or compartment and the other outside the body, i.e. they are transcutaneous devices because they cross the skin during placement and/or in use. Identical problems occur placing devices from one compartment to another within the body, e.g. placing a catheter from the subcutaneous tissue area into the blood vessel when the catheter is to be used in conjunction with a subcutaneous vascular access portal. Transcutaneous devices are included when the term intercompartmental is used herein. Several methods have been used to place such devices, each with advantages and problems.
Perhaps the earliest and most straight forward approach is to surgically cut an opening through from one compartment into the other compartment, insert the device, then sew up the surgical wound which will self-repair and heal around the device. One example of this is known as the "cut-down" placement of a catheter. In this procedure, the physician cuts through the skin with a scalpel down to a blood vessel, nicks the vessel, inserts the catheter tip through the nick and into the vessel, advances the catheter tip to the desired location, sews the vessel wall tightly around the catheter, and finally sews up the skin incision. One advantage of this method is that generally any device can be placed practically anywhere in the body. Disadvantages include the need for a skilled, highly trained physician as well as specialized medical facilities to perform the procedure safely, and the trauma, disruption and infection risk to the many tissues involved.
To address these disadvantages, percutaneous methods have been developed to create small openings and place such devices through them with minimal tissue disruption, using specialized placement assist devices to reduce the need for skilled personnel and elaborate operating room facilities. One such device is a solid dilator. The key features include a tapered end which will spread aside the tissues at a puncture site as the dilator is advanced through tissue from one compartment to another compartment and a hollow interior passage through which the device can be advanced once the dilator has created the opening. The dilator is then removed over the end of the device. A succession of gradually larger-sized dilators are sometimes used to create a large hole through which a relatively large device can be inserted. If the device is sufficiently firm, stiff and unyielding, the dilator may be used to pre-spread the tissue and create the opening, then withdrawn completely and the stiff device quickly inserted through the opening thus created before it has a chance to reclose.
However, many medical devices, including pacer leads, catheters and hydrocephalic shunts, must be made extremely soft and supple or else they will damage the body during long indwelling times resident in internal body compartments. Such devices, which are too soft and supple to respread such a predilated opening, must be inserted through the dilator or some other device which is stiffer. Such a device may be the one which has created the opening or it may be a second device which can hold open such a pre-dilated opening created in the tissue. However, the maximum diameter of the device to be placed can be no larger than the minimum diameter of the interior passage inside the solid dilator or other hold-open device because of the need to ultimately slide the dilator or hold-open device off over the device that is being placed.
This latter requirement poses a problem for many devices. For example, both catheters and pacemaker leads are devices which terminate (at the end outside the body) in connectors which, for effective use, need to be much larger than the device (catheter or lead) itself. Pacemaker leads may actually be pre-attached to the pacemaker. To place such a device with a sOlid dilator, the dilator must be made with an internal passageway as large as the largest part of the device, in this case, the terminating connector. This creates problems. The hole through the tissue must be much larger than that required to accommodate passage of a catheter tube or a pacemaker, creating tissue trauma and disruption. The required hole may be so big that it cannot be created by spreading or pushing aside the tissue, but can only be cut surgically with a scalpel, thus requiring a return to the technique less preferred. When the thin part of the device is advanced through the dilator, there will now be much empty space between the device and the walls of the internal passageway of the dilator, creating a pathway for unintended and undesirable fluids to pass between compartments during the placement procedure. One example of this could be an air embolism entering a blood vessel between a small catheter and an oversized dilator during a catheter placement procedure, a potentially life-threatening complication.
A device which solves this problem is a peel away sheath, often used in conjunction with a solid dilator. The peel away sheath is a very thin-walled, usually cylindrical device that is placed in position so that it provides a communicating passageway through the tissue. This is often accomplished by fitting the sheath tightly over a solid dilator, advancing both devices through the tissue together as a unit, then removing the solid dilator from inside the sheath, leaving the sheath alone in the desired position acting to hold the penetrated site in an open condition. Then the catheter tube or other instrument is advanced through the sheath into the desired position. The peel away sheath is capable of being readily and reliably pulled apart lengthwise into two pieces. This allows it to be made as small as the intercompartmental portion of the device, then removed by being pulled apart despite the presence of a (usually enlarged) connector or other end fitting. Proximal handles on the sheath are generally provided to facilitate grasping and tearing.
Two types of peel away sheaths are known in the prior art. U.S. Pat. No. Re 31,855, discloses a sheath, preferably of polytetraflourethylene material, that has an internal molecular orientation which tears easily in a lengthwise direction and with great difficulty in crosswise or oblique directions. This prevents an incomplete lengthwise tear which would result in an inability to remove the sheath. This sheath is furnished to the user already provided with pre-started, diametrically opposed longitudinal proximal tears. This sheath has the advantage of producing reliable tears. The tearing motion itself is easily performed by hand without the aid of a mechanical tearing device, is easily controlled, and has a good, smooth "feel" in use. Disadvantages include the severe limitation imposed on the materials that may be used for construction, whiCh can only be materials capable of being fabricated with an oriented molecular structure. This makes it difficult to reliably attach handles, difficult to consistently fabricate a well-tapered tip to facilitate advancing the sheath through the tissue over the dilator, and difficult for the sheath to resist kinking when the sheath cylinder is bent while in use.
U.S. Pat. Nos. 4,166,469, 4,243,050 and 4,345,606 disclose a sheath longitudinally scored or perforated on opposite sides. The result is a cylindrical sheath with two weakened lines which run lengthwise on opposite sides of the cylinder. It relies for its operation on the thinned, scored or perforated regions being mechanically weakened and, therefore, less resistant to tearing than the rest of the sheath cylinder, causing the tear, once started, to propagate along the weakened region and hence in a lengthwise fashion only. Advantages include a broader selection of materials permitting somewhat easier handle attachment, easier tip fabrication and better kink resistance. However, the same material is still used throughout, thus requiring that the material selected to fabricate the sheath possess properties which are a compromise between the above-mentioned factors and longitudinal shear strength. Other disadvantages also include the difficulty of consistently fabricating a sufficiently weakened area to produce 100% consistent tears, less well-controlled and smooth tears and tears that have poorer "feel".